Differential refractometer



July 13, 1948. R sTAMM ET AL 2,445,044

DIFFERENTIAL REFRACTOMETER Filed Nov. 9, 1945 4 2 Sheets-Sheet 1 INVENTORS ROBERT F. .5'774MM, H I THO/W45 M45 MFA,

ROBE/P7 a. 54/P/V5'5. awn

i452 EFV/Ff/i BY W ATTORN 4 July 13, 1948.

R. F. STAMM ETAL 2,445,044

DIFFERENTIAL REFRACTOME' IER Filed'Nov. 9, 1945 2 Sheets-Sheet 2 IPOEERT 5. EAR/V5.5, BY C'H/YPEJ/RSTV/(F,

/ I I A ATTORNEY Patented July 13, 1948 DIFFERENTIAL BEFRACTOMETER Robert Franz Stamm, Stamford, Thomas Mariner, South Norwalk, Robert Bowling Barnes,

Stamford, and Charles Rule Stryker, Greenwich, Conn., assignors to Amerlcan'Cyanamid Company, New York, N. Y., a corporation of Application November 9, 1945, Serial No. 627,638

- Claims.

This invention relates to an improved diilferential reiractometer.

It is often important to measure diflerences of refractive index of two solutions. For example;

the problem is presented when changes in refractive index must be known in order to determine molecular weightby means of light scattering.

It is necessary to measure changes in refractive inde; to the ili'th decimal place and it is desirable a monochromatic beam from a suitable slit placed at the principal focus of a collimating lens which renders the light parallel before it is passed through three prisms in series, with dispersions opposed, reflected by a mirror, and again passed through the same three prisms and finally imaged by the samelens on a split photoelectric device with a minute central dividing line between the two halves. The three prisms are liquid prisms preferably arranged in a manner similar to a Wernicke prism, with one liquid in the middle prism and the otherliquid or solution in the two end prisms. In such an optical system, a small difference in refractive index existing between the liquid in the central prism and the liquid in the two end prisms will cause an angular shift oi the beam traversing the prism system. The

sine of the angle islproportional to the refractive index d'iflerence and for small angles, the angle itself can replace thesine. with a fixed slit source of light, the lateral displacement oithe image of the sht will be proportional to the refractive index dlflerence. The two halves of the photoelectric device are connected to a diiierential galvanometer of conventional design or to the input oi'a differential electronic amplifier when amplified readings are desired.

The most accurate use of the apparatus is as a null instrument. In other words, an instrument in which the photocell or similar split radiatlondetector is brought back to the point where the slit image is on the scratch or dividing line I 2 between the two halves. The amount of movement of the photocell would then be a measure of the change in refractive index. In order to be able to read this change with an accuracy in the sixth decimal place, the motion would have to be accurate to 0.00035 cm. with the optical system of a typical model with a 50 cm. focal length lens system and a 120 central prism. Such an accuracy is possible with the finest micrometer screws but presents a serious problem in machine design. It is not excluded from the broader scope of the present invention but we prefer a difierent and more rugged modification. The preferred modification proceeds on the proposition that the measurement is displacement of the slit image with respect to the dividing line of the split radiation detector. Since we are concerned only with relative motion it makes no difference whether the image is moved and the photocell remains still, or whether the image remains still and the photocell is moved.

A very accurate method of producing a lateral displacement of the image is to pass the beam through a parallel glass-plate with optically flat surfaces, which plate can be rotated about an axis at right angles to the beam. The lateral displacement of the image produced by passing through a parallel plate of this type is substantially proportional to the tangent of the angle through which the plate is turned for angles up to ,-and by using a suitable lever arm a very accurate device can be produced which requires .eter;

This modification is therefore preferred and will be described in detail in conjunction with the drawings, in which:

Fig. 1 is a plan view of a complete refractom- Fig. 2 is a side elevation;

Fig. 3. is an enlarged detailed. sectional-view along the line 3-3 of Figs. 2 and 4;

Fig. 4 is a vertical detailed section along the line 44 of Figs. 1 and 5;

Fig. 5 is a. vertical section along the line 55 of Figs. 3 and 4; and I Fig. 6 is a diagram of the optical path through the instrument from slit to photocell. I The instrument is provided with a source of illumination, preferably a high pressure mercury are I, in a suitable housing 2, supported by the main framework of the instrument. Light from the arc passes through a small window in the housing into a tube portion which carri s filters .3 and 4 to absorb provided with an enlarged radiation except in one of the lines of the mercury are. preferably the green line of wave length,

The monochromatic radiation leaving the illters passes through a lens which images the arc on a slit I. The beam from the slitpasses on through a second tube I provided with a box-like enlargement I and a planoconvex lens "at its end; This lens transforms the beam into parallel light which passes through a triple prism Ii consisting of a central prism 20 and two end prisms 2i, strikes a mirror i2 and is reflected backthrough the prisms. The prisms are pro-- vided. with inlets and outlets 42 which permit introducing any desired liquid into any one of the three prisms. Any diflerence in refractive index between the fluid in prism 20 and the fluid in prisms 2| will produce a deviation of the beam after it has been reflected back through the prisms. This deflected beam strikes a mirror II in enlargement 8 and is reflected through a transparent plate It onto. a split photocell if provided with a very narrow dividing scratch 26 lateral. displacement of the image proportional (Figs. 1 and 6) The position of the photocell is such that the slit 1 is accurately imaged in the plane of the photocell surface. This requires a slightly longer path from the mirror l2 to the photocell il than from the slit I to the mirror, because of theeffect of the beam passing through the plate I8.

The two halves of the photocell are connected through wires 24 and to a differential conventional galvanometer of conventional design 4i. They may, of course, be connected to any other suitable differential voltage indicator or recorder. The construction of indicators and recorders forms no part of the present invention, but it is an advantage that conventional devices maybe used without modification. Since the two halves of the photocellare connected so that the currents oppose each other, the instrument will read zero' when the image is substantially centered on the scratch. When the sensitivities of the two halves are exactly equal, the null reading will result when the slit image is exactly centered on the scratch. If, as may occur, the two photocell halves differ slightly in sensitivity. the null reading will occur when the slit image is substantially centered but not exactly. there being slightly more illumination on the photocell half having the lower sensitivity.

The plate It is mounted on a shaft l8 centered where the beam is located at zero deflection. This shaftextends down through a bearing 31 in the bottom of the main framework of the machine (Fig. 4). On its bottom end there is clamped a rigid arm I! provided with a thin shoe 38 at its end made of Stellite or other hard against the two sides of the Stellite shoe. These yokes fit in recesses in a block 2! rigidly fastened to a carriage 25 which moves horizontally in material. Two Stellitepins 22 in yokes I! bear guides 22 attached to the main framework of the machine (Fig. 4). One of the yokes 38 presses against a spring 28 in the recess so that the pins are always in close but slidable contact with the Stellite shoe at the end of the arm I. The details of this portion of the device are shown in Fig. 5.

The carriage 2B is provided with a depression directly above the pins 22 in which there is a hardened steel ball 28 which projects into a corresponding depression on a threaded sleeve 21 through which a micrometer screw 2' passes. This screw is mounted in bearings I and carries to refractive index difference. The parallel plate, when inclined to the beam, will produce an equal and opposite'lateral displacement which is proportional to the tangent of the angle of plate inclination. Consequently, this tangent arm reads a quantity directly proportional to refractive index difference. Changes in refractive index are therefore proportional to the revolutions of the micrometer screw 29, and hence can. be read on the dial 40. Because a large number of revolutions will normally be needed,. a revolution counter 33 is provided which is of conventional design- The dial 40 has graduations which permit reading to the sixth decimal place when the total length of .beam travel from slit I to mirror I2 is about 50 cm. and the length of arm I! is about 10 cm.

Before the apparatus is operated it is calibrated by putting the same liquid in the three prisms and adjusting the mirrors until the beam image is on the dividing line between photocell halves as indicated by a-null reading. Then two liquids of known refractive index difference are used in the prism assembly and the plate is rotated to produce the same null reading. The two readings sufllce to calibrate the dial so that it reads a quantity directly proportional to refractive index difference. The instrument does not ordinarily change during use as good thermostatting is effected by having the prism enclosed in a housing I4 which is in turn in a thermostatic bath i2.

If desired the photocell may be moved with a very fine micrometer screw instead of using the plate l6,but this modification is less accurate unless a micrometer screw of extraordinary fine pitch is used. a

In the claims the term indicating means will be used in a generic sense to include means which indicate and record and also means which indicate only.

In the drawings, there has been illustrated a comparatively narrow slit as a source of the monochromatic light beam. The use of a moderately narrow slit presents some advantages from the standpoint of sharpness of the null reading on the indicating device, however, it is not essential that a narrow slit be used as the source. Any shape of source which will give a sharp image symmetrical with respect to a center line imageable on the gap between photoelectric devices is usable because a null reading can be obtained even though a substantial portion of both halves of the photoelectric device are illuminated provided the distribution of area of the image is symmetrical. Change in indicator reading on approach to the null point is of maximum sharpness with a narrow slit, the image of which i is only a little wider than the gap or scratch dividing the two sides of the photoelectric device and this is therefore the preferred modification of the present invention although it is not limited thereto.

We claim:

1. A diflerential refractometer comprising in combination and in optical alignment 9. source of monochromatic light, a triple prism of the Wernicke type, consisting of two end prisms and a center prism, the prisms being provided with meansthrough which liquid may be introduced, a mirror behind the triple prism positioned to reflect the beam passing therethrough back through the prism at a slight angle from the beam incident on the prism, a photoelectric device having a sharply bounded sensitive area, means for forming a sharply focused image of the source on the plane of the photoelectric device with a boundary line parallel to the boundary line of the photoelectric surface, an electrical current indicator associated with said photoelectric device, adjustable means. spaced from the Wernicke prism for producing relative movement between the photoelectric device and the image of the source across said boundaries and quantitative indicating means actuated by said means for producing relative movement.

2. A difl'erential reiractometer comprising in plane of the photoelectric device with a boundary line parallel to the gap, current indicating means associated with said split photoelectric device connected to the two sensitive areas in opposition and capable of giving a null reading when the slit is substantially imaged on the gap separating the sensitive areas of the photoelectric device, adjustable means spaced from the Wernicke prism for producing relative movement across the gap between the photoelectric device and the image of the slit, and quantitative indicating means actuated by said means for producing relative movement.

3. A differential refractometer comprising in combination and in optical alignment a slit source of monochromatic light, a triple prism of the Wernieke type, consisting of two end prisms and a center prism, the prisms being provided with means through which liquid may be intro-' duced. an autocollimating mirror behind the triple prism positioned to reflect the beam passing therethrough back through the prism, at a slight angle from the beam incident on the prism, a split photoelectric device having sensitive areas separated by a narrow straight gap, means including said mirror for forming a sharply focused image of the source on said gap, a transparent plate having front and rear surfaces parallel to each other in the path of the reflected beam between the prism and the split photoelectric device, this transparent plate being rotatable about an axis which is at right angles to the reflected beam and parallel to the gap and substantially passes through the optical axis of said beam when the source is imaged on the gap, indicating means associated with said split photoelectric device connected to the two current sensitive areas in opposition and capable of giving a null reading when the slit is substantially imaged on the gap separating the sensitive areas of the photoelectric device, means for rotating the transparent plate to deflect the image of the source at right angles to the gap separating the sensitive portions of the photoelectric means, and indicating means associated therewith and capable of measuring -a quantity proportional to the amount of deflection of the source image.

4..A device according to claim 3 in which the indicating means associated with the means for rotating the transparent plate, gives an indication proportional to the tangent of the angle throughwhich the plate is rotated, said means comprising an arm capable of rotating the'plate about its axis, and micrometric means slidably attached to said arm and capable of linear movement in a direction such that the linear movement is proportional to the tangent of the angle of rotation of the plate. and measuringmeans for indicating the extent of said movement.

ant-anchors crrap The following references are of record in the die of this patent:

UNITED STATES PATENTS Number name Date 830,225 Haber Sept. 4, 1906 1,421,342 Logan Oct. 2a, 1023 a roman Plmmrs Number Country Date 11,200 Great Britain 190s 

